I think getting a tattoo of your boxer is a great idea and the picture of Z is fantastic.
It would definately have to be done by the right artist though as it would be a nightmare if you weren't happy with the finished result or if it ended up looking more like a bulldog than a boxer.
It's true that bonier areas of the body are more painful when tattooed than others, because I've got one on my buttock and one on my rib cage and the one on my ribs was way more painful to have done. :(
Your shoulder is quite bony and could be painful, but you can probably have the work done over one or two sittings so you wouldn't have to suffer for too long in one go.
One more thing, the guy who did my second tattoo on my ribs said that in his experience women are much better at getting tattoos than men who can sometimes be big babies about the pain. Of course we already knew that!